This invention relates generally to sensors for detecting mechanical vibrations such as sonic waves that propagate through the earth or through structures formed of solid materials. In particular, this invention relates to sensors that include interferometers to detect mechanical vibrations.
There are numerous applications of mechanical vibration sensors. One application of interest is the geophone, which may be used to detect vibrations resulting from vehicles traveling across the ground or on a highway. It is possible to determine the types of vehicle and the number of each type of vehicle in a convoy or the like by using a geophone to detect vibrations that propagate through the earth and then analyzing the resulting electrical signals.
Commercial applications of geophones include exploration for oil and minerals and testing rotating machinery for wear. In exploring for oil, for example, a plurality of geophones are placed in an array at various locations on the surface of the earth and in bores drilled into the earth. An explosive charge is used to create a shock wave, which is detected by the geophones. Analysis of the data from the geophone array then is used to determine the structure of the earth in the vicinity of the explosion and the geophones.
Mechanical vibrations in rotating machinery such as water or gas driven turbines used in electric power generation may be used to measure the amount of wear in bearings, gears and the like.
Prior art sensors for these applications typically use either piezoelectric ceramics or other mechanical means to sense the applied field. Prior art devices are electrically active in that electric power must be applied to produce an output signal. This electrical downlink can prove to be undesirable to due electromagnetic interference and undesired detection. In addition, the data that is collected and sent back to the remote location is also subject to the bandwidth limitations of twisted pair or coaxial cables. The output from these devices is electrical. Therefore, to transmit the sensed information via an optical telemetry system, a converter must be used to change the information from electrical to an optical signal, which adds expense and extra components.